1. Field of the Invention
The present invention relates to a female substrate used in the production of microprobe tips for scanning tunneling microscopes and atomic force microscopes for detecting significantly weak forces. The present invention also relates to a method for making the female substrate. Further, the present invention relates to methods for making a microprobe tip and a probe using the female substrate. In particular, the present invention relates to a female substrate used in the production of microprobe tips and a method for making the same, in which microprobe tips having substantially the same curvature radius and being suitable for use in the above-mentioned microscopes can be produced on the same or different substrates with a high mass production efficiency.
2. Description of the Related Art
Since the development of a scanning tunneling microscope (hereinafter referred to as "STM") (G. Binning et al., Phys. Re. Lett., 49, 57 (1983)), direct observation of real spatial images on the electronic structure of atoms at the surface of a conductor has been achieved at a high resolution regardless of whether it is a single-crystal or amorphous material. The STM detects a tunnel current that occurs between a metal tip and the conductive material being observed when the conductive material approaches the tip at a distance of approximately 1 nm. The current changes exponentially with the distance between them. When the tip is scanned on the conductive material so that the tunnel current has a constant value, the surface structure of the real space can be observed with a resolution at the atomic level. Although STMs have formerly been used for analysis of only conductive materials, they have also been used recently for structural analysis of thin insulating layers formed on the surfaces of conductive materials.
The STM can detect a microcurrent flow, hence a medium can be observed with a low amount of electric power without damage to the medium. Since the STM can work in the atmosphere, it has been applied to various fields, for example, observation at the atomic or molecular level of semiconductors and polymers, micro-fabrication process (E. E. Ehrichs, "Proceedings of the 4th International Conference on Scanning Tunneling Microscopy/Spectroscopy", 89, S13-3), and information recording/regenerating devices. Simultaneous drive of many probes, that is, the use of multiple tips is proposed for application of the STM to information recording/regenerating devices, hence production of a plurality of tips having substantially the same curvature radius on a substrate is required. The atomic force microscope (hereinafter referred to as "AFM") detects a repulsive force or attractive force working between the tip and the surface of a material, and permits observation of a topographical image of the surface whenever the material is a conductor or insulator. The AFM uses a thin film cantilever provided with a microprobe tip at the free end of the cantilever. Also, in AFM measurement, microprobe tips having the same curvature radius must always be used in order to secure high reproducibility.
A typical method for making such microprobe tips is disclosed in U.S. Pat. No. 5,221,415, in which a microprobe tip is formed by anisotropic etching of single-crystal silicon by means of a semiconductor fabrication process. As shown in FIG. 1, a pit 518 as a female mold for a microprobe tip is formed on a silicon wafer 514 covered with silicon dioxide masks 510 and 512 by an anisotropic etching process, and the silicon dioxide masks 510 and 512 are removed. Both surfaces of the silicon wafer 514 are covered with silicon nitride layers 520 and 521. The upper silicon nitride layer 520 has a pyramidal pit 522. After the upper silicon nitride layer 520 is patterned to form a cantilever, the silicon nitride layer 521 on the bottom surface is removed. A glass plate 530 provided with a sawcut 534 and a chromium layer 532 is bonded to the upper silicon nitride layer 520, and the silicon wafer 514 is removed by etching. As a result, a probe consisting of a microprobe tip and a cantilever, which are composed of silicon nitride, is formed on a mounting block 540. Finally, a metal film 542 as a reflection film for an optical lever-type AFM is formed on the bottom surface. In this method, the curvature radius of the silicon nitride tip can be controlled by forming a silicon dioxide film by thermal oxidation on the pit as the female mold after removal of the silicon dioxide masks 510 and 512 (S. Akamine and C. F. Quate, "Low temperature thermal oxidation sharpening of microcast tips", J. Vac. Sci. Technol. B10(5), September/October, p. 2307 (1992)).
In another method for tip production, as shown in FIG. 2A, a tip 613 is formed by side etching of a silicon substrate 611 using a disk-shape mask 612 provided on the silicon substrate 611 (O. Wolter, et. al., "Micromachined silicon sensors for scanning force microscopy", J. Vac. Sci. Technol., B9(2), March/April, pp. 1353-1357 (1991)). Alternatively, as shown in FIG. 2B, a tip 623 is also formed by evaporating a conductive material 625 placed on a resist 622 in the orthogonal directions and depositing it onto a rotating substrate 621 through a resist opening 624 (C. A. Spindt, et. al., "Physical properties of thin film field emission cathode with molybdenum cones", J. Appl. Phys., 47, pp. 5248-5263 (1976)).
Although the conventional method shown in FIG. 1 permits the production of microprobe tips with high reproducibility, the controllable curvature radius of the tips is limited to a narrow range from 10 nm to 40 nm.
In the methods shown in FIGS. 2A and 2B, although tips having various curvature radii can be produced, severe process control is required in order to maintain constant conditions for silicon etching, resist patterning and evaporation of the conductive material. It is difficult to produce a plurality of tips with high reproducibility of height and curvature radius.